What can be/should be done with PSD files included with Themes


I understand that PSD files are created by designers, then sliced, then exported and css etc. is created around it to create a theme. Eventually I may try to build a theme myself, but I have a more conceptual/best practices question that I basically just want to make sure I'm not missing b/c it's right in front of my face. Here it is:

To customize a Wordpress theme, is it best to customize the provided PSD and reexport or whatever needs to be done? If so, any documentation/resources would be apprecited.

I typically just use the custom css, etc. to do this and I'm just wondering if I'm missing something obvious here.